As an example of a transmission-type screen for use in a rear projection-type television, a transmission-type screen including a diffusion sheet and a Fresnel lens sheet has conventionally been known. FIG. 7 is a cross-sectional plan view of a conventional transmission-type screen. In FIG. 7, the reference number 31 indicates a transmission-type screen, 32 indicates a Fresnel lens sheet, and 33 indicates a diffusion sheet.
The Fresnel lens sheet 32 has a concentrically-formed Fresnel lens on the side of its emergence surface. The Fresnel lens has a plurality of concentric Fresnel lens surfaces which are formed oblique with respect to an incidence surface. A cross-section of the Fresnel lens has a saw-tooth appearance.
FIG. 8 is an enlarged cross-sectional view of the diffusion sheet 33. As shown in FIG. 8, the diffusion sheet 33 has a plurality of V-shaped grooves 41 each having a V-shaped cross-section, which are disposed on the side of the emergence surface (upper side in FIG. 8) Each groove 41 is extended in a height direction of the diffusion sheet 33 (in a depth direction of the drawing sheet of FIG. 8). Respective parts sandwiched between the V-shaped grooves 41 form a plurality of ribs 42 each having a trapezoidal cross-section. A side surface 43 of each rib 42 provides an inclined surface of each V-shaped groove 41. A light incident on the side surface 43 is emitted from the diffusion sheet 33 with its advancing direction changed by a reflection at the side surface 43.
The transmission-type screen 31 is positioned such that the Fresnel lens sheet 32 faces a projection device (not shown), and such that the diffusion sheet 33 faces a viewer (not shown). The projection device projects image light toward the transmission-type screen 31, and the viewer observes a projected image on the transmission-type screen 31. At this time, the image light projected by the projection device is inflected by the Fresnel lens surfaces of the Fresnel lens sheet 32, so that the light is adjusted to be in a substantially perpendicular direction to the diffusion sheet 33. The image light adjusted to be in a substantially perpendicular direction is then incident on the side surfaces 43 and top surfaces 44 of the ribs 42. Of this light, the image light incident on the top surfaces 44 of the ribs 42 hardly changes in its advancing direction, and is emitted from the diffusion sheet 33 in a direction substantially perpendicular to the transmission-type screen 31. On the other hand, the image light incident on the side surfaces 43 of the ribs 42 is reflected by the side surfaces 43 to change its advancing direction, and is then inflected by the top surfaces 44 of the ribs 42 to be emitted from the diffusion sheet 33. In this way, some of the image light (image light incident on the side surfaces 43 of the ribs) projected by the projection device is diffused in a width direction of the transmission-type screen 31 to be emitted from the diffusion sheet 33. Thus, the viewer can observe an image with a wide viewing angle in the width direction.
Image light emitted from the conventional transmission-type screen is composed of three groups of light directed in three directions; i.e., a group of light which is directly incident on each top surface 44 of the ribs to be emitted in a perpendicular direction to the transmission-type screen 31; a group of light which is reflected by each side surface 43a on one side of the ribs to be emitted obliquely to the other side of the ribs; and a group of light which is reflected by each side surface 43b on the other side of the ribs to be emitted obliquely to the one side of the ribs. Thus, as shown in FIG. 9, a gain curve of the image light emitted from the transmission-type screen 31 is provided. Herein, the gain curve means a curve formed by plotting ratios of an amount of emergent (outgoing) light to incident light with respect to the respective viewing angles. (Gain is calculated as follows: Light is radiated from the rear side of the screen. An angular distribution of brightness of light emitted forward is measured. An illuminance on the incident surface of the screen and the respective brightness are applied to a relational expression of gain G=π×brightness (cd/m2)/illuminance (lux). An illuminance meter such as “ANA-F12 type” manufactured by Sibata Scientific Technology Ltd. may be used, while a luminance meter such as “LS-110” manufactured by Minolta Co., Ltd. may be used.)
As shown in FIG. 9, the gain curve of the transmission-type screen 31 has three peaks, that is, a peak at 0° (perpendicular to the screen), a peak at +25° (inclined 25° rightward to the width direction of the screen), and a peak at −25° (inclined 25° leftward to the width direction of the screen). Thus, an image (spot image) projected on the transmission-type screen 31 is brightly seen when observed from the directions of 0°, +25°, and −25°. However, when observed from other directions, the image seen is extremely dark (or cannot be seen at all). Namely, when the viewer observes an image while moving in the width direction of the transmission-type screen 31, the image is observed in an alternating manner of “bright”, “dark”, “bright”, and so on. This feels strange to the viewer. When a wide image is observed, the viewer also feels strange because of a significant uneven brightness of the center of the image and both ends thereof. In short, the quality of the transmission-type screen is quite low.